1. Field of Invention
The present invention relates to a surface light source device used for backlighting an object-to-be-illuminated such as LCD panel or advertising panel, for example, in portable telephones, portable mobile terminal devices, car navigation devices, personal computers, video cameras, digital still cameras, electronic pocket note books or LCD-TV sets, and to displays provided with the surface light source device.
2. Related Arts
FIG. 14 shows an example (first prior art) of conventional surface light source device including a light guide plate, primary light source supplying light to the light guide plate and light control member (for example, prism sheet) disposed along an emission face of the light guide plate as basic components, wherein the surface light source device is used in a conventional image display.
Referring to FIG. 14, surface light source device 102 is arranged for backlighting LCD panel (image displaying panel) 103 of personal computer or the like. A side end face (minor face) of light guide plate 104 provides incidence face 105 receiving light from LED (primary light source) 106. A major face provides an emission face 110 for emitting light. Light control member 111 is interposed between emission face 110 of light guide plate 104 and LCD panel 103. Light emitted from emission face 110 is transmitted and redirected by light control member 111 as to come close angularly to a generally frontal direction (normal direction of emission face 110 (Z-direction), illumination LCD panel 103.
Back face 107 of light guide plate 104 is provided with a great number of prismatic projection rows 118 each of which has a triangle-like cross section. Projection rows 118 run in a direction (Y-direction) perpendicular to incidence face 105, performing a light deflecting function. That is, projection rows 118 deflect light propagating within light guide plate 104, causing the light to have a direction near to a normal direction of emission face 110 in an imaginary plane parallel to incidence face 105 and perpendicular emission face 110.
On the other hand, light control member 111 has an inner face (i.e. a face directed to emission face 110) provided with a great number of prismatic projection rows 115 each of which has a triangle-like cross section. Projection rows 115 run generally in a direction (X-direction) parallel to incidence face 105, performing another light deflecting function. That is, projection rows 115 deflect light emitted from emission-face 110, causing the emitted light to have a direction near to a frontal direction (Z-direction) of emission face 110 in an imaginary plane perpendicular to incidence face 105 and perpendicular emission face 110.
As a result, LCD panel 103 is efficiently supplied with light so that incidence occurs to a back face thereof in a generally vertical direction, thereby providing a high display brightness.
However, it is known that image displays of the above type are subject to a problem that stripe pattern like Moire-stripe is apt to appear on a screen of LCD panel 103. Needless to say, such stripe pattern reduces display quality.
FIG. 15 illustrates an example (second prior art) improved to avoid this problem, being disclosed in Document 1 cited below.
As illustrated in FIG. 15, this arrangement employs an improved surface light source device 102. An improved point is that an oblique orientation is applied to prismatic projection rows 115 on light control member 111. In other words, projection rows 115 run in a direction inclined (0°<inclination angle θ<90°) with respect to incidence face 105 of light guide plate 104. Such an inclined orientation gives a reduced tendency of appearance of stripe pattern like Moire-stripe.
Document 2 cited below discloses another improved art (third prior art), being illustrated in FIG. 16. This prior art employs surface light source device 202 having light guide plate 204. Light guide plate 204 has incidence face 205 provided with a plurality of projections 212 or grooves 224 ruining in a thickness direction.
Each projection 212 or groove 224 has a plan-shape (shape as viewed from just above emission face 210 of light guide plate 204) which is symmetric with respect to a normal direction of incidence face 205. Light from LED 206 enters into light guide plate 204 through projections 212 or grooves 224.
This entering involves an angular expansion of light as compared with a case where incidence face 205 were a minor face (namely, neither projections 212 nor grooves 224 are formed).
As a result, an area of a triangle-like dark region (hatched region) appearing on emission face 210 in the vicinity of incidence face 205 is reduced and an effective emission area of light guide plate 204 is enlarged. In addition, a uniformalized emission brightness is provided by emission face 210.
If projections 212 or grooves 224 are formed on incidence face 205 as shown in FIG. 16 is applied to light guide plate 104 of surface light source device 102, Moire pattern is reduced because prismatic projection rows 115 of light control member 111 run in a direction inclined with respect to incidence face 105.
However, there rises a difference in brightness between right-side-end L1 and left-side-end L2 of a sector-like incident light in the vicinity of incidence face 205, as illustrated in FIG. 16. This hinders emission brightness from being uniformalized.
DOCUMENT 1:Jikkai-Hei P-A) 5-25426
DOCUMENT 2:Tokkai (JP-A) 2002-196151